U.S. patent number 10,050,729 [Application Number 15/088,263] was granted by the patent office on 2018-08-14 for mobile station and reception quality measurement method.
This patent grant is currently assigned to Panasonic Intellectual Property Corporation of America. The grantee listed for this patent is Panasonic Intellectual Property Corporation of America. Invention is credited to Ayako Horiuchi, Masayuki Hoshino, Seigo Nakao.
United States Patent |
10,050,729 |
Horiuchi , et al. |
August 14, 2018 |
Mobile station and reception quality measurement method
Abstract
Received Signal Strength Indicator (RSSI) is measured accurately
even in a case where a discovery signal is transmitted. A receiver
receives a plurality of subframes, at least one of which includes a
discovery signal, and a measurer measures Reference Signal
Reception Power (RSRP) using a first resource in which the
discovery signal is mapped, measures RSSI using a second resource
different from the first resource for which the discovery signal is
mapped, and calculates Reference Signal Reception Quality (RSRQ)
using RSRP and RSSI.
Inventors: |
Horiuchi; Ayako (Kanagawa,
JP), Hoshino; Masayuki (Kanagawa, JP),
Nakao; Seigo (Singapore, SG) |
Applicant: |
Name |
City |
State |
Country |
Type |
Panasonic Intellectual Property Corporation of America |
Torrance |
CA |
US |
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Assignee: |
Panasonic Intellectual Property
Corporation of America (Torrance, CA)
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Family
ID: |
53477906 |
Appl.
No.: |
15/088,263 |
Filed: |
April 1, 2016 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20160218816 A1 |
Jul 28, 2016 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCT/JP2014/006063 |
Dec 4, 2014 |
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Foreign Application Priority Data
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Dec 25, 2013 [JP] |
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2013-267112 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L
5/005 (20130101); H04B 17/318 (20150115); H04W
48/12 (20130101) |
Current International
Class: |
H04L
1/00 (20060101); H04B 17/318 (20150101); H04L
5/00 (20060101); H04W 48/12 (20090101) |
Field of
Search: |
;370/252 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2014-204278 |
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Oct 2014 |
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JP |
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2015-065607 |
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Apr 2015 |
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JP |
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Other References
International Search Report of PCT application No.
PCT/JP2014/006063 dated Feb. 3, 2015. cited by applicant .
3GPP TR 36.872 V12.0.0, "Small cell enhancements for E-UTRA and
E-UTRAN-Physical layer aspects (Release12)" Sep. 2013. cited by
applicant .
3GPP TR 36.842 V1.0.0, "Study on Small Cell Enhancements for E-UTRA
and E-UTRAN-Higher layer aspects (Release12)" Nov. 2013. cited by
applicant .
3GPP TS 36.214 V11.0.0, "Physical layer; Measurements (Release11)"
Sep. 2012. cited by applicant.
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Primary Examiner: Pham; Chi H
Assistant Examiner: Boakye; Alexander O
Attorney, Agent or Firm: Seed IP Law Group LLP
Claims
What is claimed is:
1. A mobile station comprising: a receiver that receives a
plurality of subframes, at least one of which includes a discovery
signal that is a signal transmitted from an OFF-state cell having
no traffic data to be transmitted and is used by the mobile station
to detect the OFF-state cell; and measuring circuitry that is
coupled to the receiver and that measures Reference Signal
Reception Power (RSRP) using a first resource in which the
discovery signal is mapped, measures Received Signal Strength
Indicator (RSSI) using a second resource in which the discovery
signal is not mapped, and calculates Reference Signal Reception
Quality (RSRQ) using the RSRP and the RSSI.
2. The mobile station according to claim 1, wherein the first
resource is a first subframe and the second resource is a second
subframe among the received plurality of subframes, and the
measurer measures the RSSI in the second subframe that is different
from the first subframe for which the RSRP is measured.
3. The mobile station according to claim 2, wherein the measurer
measures the RSSI using all of a plurality of symbols included in
the second subframe.
4. The mobile station according to claim 2, wherein the measurer
measures the RSSI using symbols that include Cell specific
Reference Signal (CRS), among a plurality of symbols included in
the second subframe.
5. The mobile station according to claim 2, wherein the second
subframe is located adjacent to the first subframe.
6. The mobile station according to claim 1, wherein the measurer
measures the RSSI using symbols that are part of a plurality of
symbols included in a subframe for which the RSSI is measured,
wherein the symbols that are used to measure the RSSI are symbols
other than a particular number of symbols in a first half portion
or a second half portion of the subframe for which the RSSI is
measured.
7. The mobile station according to claim 1, wherein the resource is
a subframe and the measurer measures the RSSI in the same subframe
as the subframe for which the RSRP is measured.
8. The mobile station according to claim 1, wherein the resource is
a resource element (RE) and the measurer measures the RSSI using an
RE other than any RE in which a discovery signal is mapped.
9. The mobile station according to claim 7, wherein the measurer
measures the RSSI using a symbol other than symbol(s) that includes
a resource in which a discovery signal is mapped.
10. The mobile station according to claim 7, wherein the measurer
measures the RSSI using a symbol that includes a Cell specific
Reference Signal (CRS), among symbols other than that include a
resource in which the discovery signal is mapped.
11. A reception quality measurement method implemented in a mobile
station, the method comprising: receiving a discovery signal that
is a signal transmitted from an OFF-state cell having no traffic
data to be transmitted and is used by the mobile station to detect
the OFF-state cell; and measuring Reference Signal Reception Power
(RSRP) using a resource in which the discovery signal is mapped,
measuring Received Signal Strength Indicator (RSSI) using a
resource in which the discovery signal is not mapped, and
calculating Reference Signal Reception Quality (RSRQ) using the
RSRP and the RSSI.
12. The mobile station according to claim 1, wherein the receiver
receives discovery signal information that indicates a subframe in
which the discovery signal is mapped; the measuring circuit detects
the indicated subframe as the first resource in which the discovery
signal is mapped.
13. The mobile station according to claim 12, wherein a plurality
of discovery signals transmitted from a plurality of OFF-state
cells are mapped in the indicated subframe.
14. The mobile station according to claim 12, wherein no traffic
data is mapped in the first resource and traffic data transmitted
from an ON-state cell to the mobile station is mapped in the second
resource.
15. The reception quality measurement method according to claim 11,
comprising receiving discovery signal information that indicates a
subframe in which the discovery signal is mapped; detecting the
indicated subframe as the first resource in which the discovery
signal is mapped.
16. The reception quality measurement method according to claim 15,
wherein a plurality of discovery signals transmitted from a
plurality of OFF-state cells are mapped in the indicated
subframe.
17. The reception quality measurement method according to claim 15,
wherein no traffic data is mapped in the first resource and traffic
data transmitted from an ON-state cell to the mobile station is
mapped in the second resource.
Description
BACKGROUND
1. Technical Field
The present disclosure relates to a mobile station and a reception
quality measurement method.
2. Description of the Related Art
In recent years, in cellular mobile communication systems, it has
become popular to provide information in a multimedia form such
that not only audio data but also still image data, moving image
data, or the like with a large data size is transmitted. In
LTE-Advanced (Long Term Evolution Advanced), an active
investigation has been made to achieve a high transmission rate
using a wireless broadband, a Multiple-Input Multiple-Output (MIMO)
transmission technique, and an interference control technique.
Furthermore, in LTE-Advanced, it is under consideration to provide
a small cell, which is a base station with low transmission power
(also referred to as e Node B (eNB)), to achieve a high
transmission rate at a hot spot. It is under consideration that a
carrier frequency is assigned to the small cell such that the
assigned frequency is different from that used in a macro cell,
which is a base station with high transmission power, (see, for
example, 3GPP TR 36.872 V12.0.0 (2013-09), Small Cell Enhancements
for E-UTRA and E-UTRAN Physical layer Aspects).
It is also under consideration to allow a mobile station (which is
also called user equipment (UE) or a terminal) to by itself connect
to a small cell. It is also under consideration to use carrier
aggregation, in which a plurality of component carriers are used,
to allow a mobile station to connect to both a macro cell and a
small cell. Furthermore, it is also under consideration to employ
dual connectivity to allow a mobile station to connect to a Master
eNB (MeNB) and a Secondary eNB (SeNB) (see, for example, 3GPP TR
36.842 V1.0.0 (2013-11), Study on Small Cell Enhancements for
E-UTRA and E-UTRAN Higher layer Aspects). In the Dual Connectivity,
a cell that manages mobility of mobile stations is called MeNB.
Other than MeNB, a cell that assigns a resource to a mobile station
is called SeNB. A mobile station is allowed to use both a resource
of MeNB and that of SeNB.
In a case where a mobile station connects by itself to a small
cell, the mobile station is likely to move to the small cell in
response to receiving a handover command from another cell. In a
case where the carrier aggregation is used, a small cell is likely
to be set as a Secondary Cell (SCell). In the case of the Dual
connectivity, a small cell is likely to be set as SeNB. In any
case, before a mobile station makes a connection to a cell, the
mobile station needs to identify the cell, achieve synchronization
with the cell, and measure reception quality between the cell and
the mobile station.
Conventionally, the cell synchronization is achieved via Primary
Synchronization Signal (PSS)/Secondary Synchronization Signal (SSS)
transmitted at intervals of 5 msec. Thereafter, a cell ID is
identified and then Radio Resource Management (RRM) is performed
using reception power of Cell specific Reference Signal (CRS) and
reception power of the whole band.
RRM is used in measurement for mobility management such as a cell
selection or the like. In RRM, Reference Signal Reception Power
(RSRP) or Reference Signal Reception Quality (RSRQ) is measured. In
a case where RSRP of an adjacent cell satisfies a predetermined
criterion, for example, in a case where RSRP of the adjacent cell
is higher by 3 dB than that of a current cell, a mobile station
makes a report of a cell ID and RSRP of this adjacent cell. Here in
a case where information on the cell ID of the adjacent cell has
been informed, the mobile station is capable of detecting the cell
using the cell ID.
RSRP is average reception power of CRS, and RSRQ is given by
N*RSRP/RSSI (see, for example, 3GPP TS 36.214 V11.0.0 (2012-9),
Physical layer; Measurements) where N is the number of resource
blocks (RBs) in a band in which Received Signal Strength Indicator
(RSSI) is measured, and RSSI is the average reception power in an
OFDM symbol. In a case where no instruction is given from a higher
layer, RSSI is measured in an OFDM (Orthogonal Frequency Division
Mutiplexing) symbol in which CRS is mapped. On the other hand, in a
case where a subframe in which RSSI is to be measured is specified
from the higher layer, RSSI is measured in all OFDM symbols in the
specified subframe. RSRP corresponds to reception power of a
certain cell and RSSI corresponds to reception power of a whole
band, and thus RSRQ indicates the ratio of the reception power of
the certain cell to the reception power of the whole band including
interference. RSRQ is a parameter including an amount of
interference varying depending on the band, and thus RSRQ is used
in comparison (interband comparison) in terms of reception quality
of a cell between different bands.
As described above, in the cell selection based on the RSRQ
criterion, a cell is selected by comparing reception quality of
cells between different bands. In this process, if traffic per band
is large, there are many cells that may cause interference, and
thus RSSI has a large value and RSRQ has a small value. On the
other hand, if traffic per band is small, there are a small number
of cells that may cause interference, and thus RSSI has a small
value and RSRQ has a large value. Therefore, in the cell selection
based on the RSRQ criterion, the difference in traffic between
bands is taken into account, which results in an increase in the
probability that a cell with a band having a low traffic is
selected. When a cell with a band having a low traffic is selected,
less interference is achieved and it is possible to use much
resource of the cell, and thus it is possible to advantageously
achieve an increase in user throughput.
SUMMARY
However, when a resource used in the RSSI measurement includes a
discovery signal, it becomes impossible to reflect an actual
traffic in RSRQ. The discovery signal is a signal transmitted from
a cell in the OFF state in which the cell has no traffic. At a
mobile station, if a resource used in the RSSI measurement includes
discovery signals transmitted from a plurality of cells in the OFF
state, the discovery signals cause an increase in measured RSSI,
and a reduction occurs in RSRQ value. As described above, if a
discovery signal is transmitted from a base station to a terminal,
it becomes difficult to accurately measure RSSI.
One non-limiting and exemplary embodiment provides a mobile station
and a reception quality measurement method capable of accurately
measuring RSSI even in a case where a discovery signal is
transmitted.
In one general aspect, the techniques disclosed here feature that a
mobile station includes a receiver that receives a plurality of
subframes, at least one of which includes a discovery signal, and a
measurer that measures Reference Signal Reception Power (RSRP)
using a first resource in which the discovery signal is mapped,
measures Received Signal Strength Indicator (RSSI) using a second
resource different from the first resource for which the discovery
signal is mapped, and calculates Reference Signal Reception Quality
(RSRQ) using the RSRP and the RSSI, the RSRQ being to be used by
the mobile station to compare reception qualities between inter
band-cells.
The present disclosure makes it possible to accurately measure RSSI
even in a case where a discovery signal is transmitted from a base
station to a terminal.
It should be noted that general or specific embodiments may be
implemented as a system, a method, an integrated circuit, a
computer program, a storage medium, or any selective combination
thereof.
Additional benefits and advantages of the disclosed embodiments
will become apparent from the specification and drawings. The
benefits and/or advantages may be individually obtained by the
various embodiments and features of the specification and drawings,
which need not all be provided in order to obtain one or more of
such benefits and/or advantages.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating a configuration of main
parts of a mobile station according to a first embodiment of the
present disclosure;
FIG. 2 is a block diagram illustrating a configuration of a base
station according to the first embodiment of the present
disclosure;
FIG. 3 is a block diagram illustrating a configuration of a mobile
station according to the first embodiment of the present
disclosure;
FIG. 4 is a diagram illustrating an example of a subframe for RSRP
measurement;
FIG. 5 is a diagram illustrating an RSSI measurement area according
to the first embodiment of the present disclosure;
FIG. 6 is a diagram illustrating an example of mapping of CSI-RS
candidates;
FIG. 7 is a diagram illustrating an RSSI measurement area in a
first example of operation according to a second embodiment of the
present disclosure;
FIG. 8 is a diagram illustrating an RSSI measurement area in a
first example of operation according to a second embodiment of the
present disclosure;
FIG. 9 is a diagram illustrating an example of mapping of
CRS/PSS/SSS;
FIG. 10 is a diagram illustrating an RSSI measurement area in a
second example of operation according to the second embodiment of
the present disclosure; and
FIG. 11 is a diagram illustrating an RSSI measurement area in a
second example of operation according to the second embodiment of
the present disclosure.
DETAILED DESCRIPTION
Embodiments of the present disclosure are described in detail below
with reference to drawings. Note that in the embodiments, similar
constituent elements are denoted by similar reference symbols, and
a duplicated description thereof is omitted.
Underlying Knowledge Forming Basis of the Present Disclosure
In introduction of small cells, it is under consideration to
introduce an OFF state in a small cell to suppress interference
from the small cell and reduce power consumption of the small cell.
When a small cell is in the OFF state, the small cell is in a "halt
state" in which no data is assigned to a mobile station. However,
if transmission of all signals from small cells is stopped, a
mobile station cannot detect any small cell. Thus, to allow a
mobile station to detect a small cell even in the OFF state, it is
under consideration to allow a small cell to transmit a discovery
signal.
The discovery signal is used for a similar purpose to the purpose
of conventional PSS/SSS/CRS. That is, the discovery signal is a
signal for identifying a cell in the OFF state, achieving
synchronization, and managing radio resources. To suppress
interference and power consumption, it is under consideration to
reduce a transmission rate (to increase a transmission repetition
period) of the discovery signal compared with the conventional
PSS/SSS/CRS. It is under consideration to transmit discovery
signals from a plurality of cells or transmission points using the
same subframe such that a mobile station detects the plurality of
cells at the same time. In designing of the discovery signal, it is
under consideration to obtain a signal used as the discovery signal
by changing a repetition period, a band, or the like of an existing
signal in the LTE-Advanced system. Candidates for the signal to be
used include Positioning Reference Signal (PRS), Channel State
Information (CSI)-RS, PSS/SSS/CRS, etc. By transmitting the
discovery signal not only in the OFF state but also in an ON state
continuously after transition is made from the OFF state to the ON
state, it becomes possible to support the conventional detection
method and the new detection method without switching the
method.
However, when a discovery signal is transmitted from a base station
to a terminal, if the discovery signal is included in a resource
used in the RSSI measurement, this makes it difficult to accurately
measure RSSI. More specifically, the discovery signal may cause an
increase in measured RSSI and a reduction in RSRQ value. The
above-described reduction in RSRQ value caused by the discovery
signal makes it difficult to select a cell with a band in which
actual traffic is low (that is, a cell in which actual RSRQ value
is large).
The inventors of the present disclosure have made investigation on
the problems described above, and have obtained knowledge described
below, which have allowed it to achieve embodiments of the present
disclosure. That is, it is possible to accurately measure RSSI by
measuring Reference Signal Reception Power (RSRP) using a resource
in which a discovery signal is mapped, measuring Received Signal
Strength Indicator (RSSI) using a resource different from the
resource in which the discovery signal is mapped, and calculating
Reference Signal Reception Quality (RSRQ) using RSRP and RSSI.
First Embodiment
Outline of Communication System
A communication system according to the present embodiment includes
a base station 100 and a mobile station 200. This communication
system is, for example, a LTE-Advanced system. The base station 100
is, for example, a base station supported in the LTE-Advanced
system. A mobile station 200 is, for example, a mobile station
supported in the LTE-Advanced system.
In the base station 100 according to the present embodiment, an OFF
state is introduced, and the OFF state and the ON state are
switched in the operation. The base station 100 transmits a
discovery signal to the mobile station 200 at least in the OFF
state. That is, the base station 100 is a cell that supports the
discovery signal.
FIG. 1 is a block diagram illustrating a configuration of main
parts of the mobile station 200 according to the present
embodiment.
In the mobile station 200, a receiver 201 receives a discovery
signal. An RSRP/RSRQ measurement unit 204 measures RSRP (Reference
Signal Reception Power) using a resource in which the discovery
signal is mapped. Furthermore, the RSRP/RSRQ measurement unit 204
measures RSSI (Received Signal Strength Indicator) using a resource
different from the resource in which the discovery signal is
mapped. The RSRP/RSRQ measurement unit 204 then calculates RSRQ
(Reference Signal Reception Quality) using RSRP and RSSI.
Configuration of Base Station 100
FIG. 2 is a block diagram illustrating a configuration of the base
station 100 according to the present embodiment. In FIG. 2, the
base station 100 includes a discovery signal information generator
101, a signal assignment unit 102, a transmitter 103, a receiver
104, and a measurement report receiver 105.
The discovery signal information generator 101 generates discovery
signal information indicating a transmission repetition period, a
transmission subframe, a transmission frequency band, a bandwidth,
or an RB (Resource Block) of the discovery signal. The discovery
signal information generator 101 outputs the discovery signal
information as a higher layer control signal to the signal
assignment unit 102.
Note that the discovery signal information generator 101 may
generate not only discovery signal information associated with the
cell managed by the base station 100 but also discovery signal
information associated with another adjacent cell. This makes it
possible for the mobile station 200 to acquire information from a
cell in terms of a discovery signal transmitted by another
cell.
The signal assignment unit 102 assigns a discovery signal to a
particular resource based on the control signal including the
discovery signal information received from the discovery signal
information generator 101. The discovery signal is assigned in a
case where the base station 100 does not perform assignment of a
transmission data signal (that is, Physical Downlink Shared Channel
(PDSCH)) (in the OFF state in which data transmission is not
performed). Furthermore, the signal assignment unit 102 assigns a
transmission data signal and a control signal to a particular
resource. Thus by assigning the discovery signal or the control
signal and the transmission data signal to the particular resource
in the above-described manner, a transmission signal is formed. The
formed transmission signal is output to the transmitter 103.
The transmitter 103 performs a wireless transmission process such
as up-converting on the transmission signal received from the
signal assignment unit 102 and transmits the transmission signal to
the mobile station 200 via an antenna.
The receiver 104 receives the signal transmitted from the mobile
station 200 via an antenna and outputs the reception signal to the
measurement report receiver 105, and the receiver 104 extracts a
reception data signal from the reception signal.
The measurement report receiver 105 extracts a measurement report,
transmitted from the mobile station 200, from the reception signal
received from the receiver 104, and outputs the extracted
measurement report to a higher layer. The measurement report is
generated by the terminal 200 using RSRP or RSRQ of the base
station 100, another base station, or a transmission point, and the
measurement report includes information used in managing movement
and connection such as a cell selection by the mobile station
200.
Configuration of Mobile Station 200
FIG. 3 is a block diagram illustrating a configuration of the
mobile station 200 according to the present embodiment. In FIG. 3,
the mobile station 200 includes a receiver 201, a signal
demuliplexer 202, a discovery signal information receiver 203, an
RSRP/RSRQ measurement unit 204, a measurement report generator 205,
a signal assignment unit 206, and a transmitter 207.
The receiver 201 receives a signal via an antenna, performs a
reception process such as down-converting, and outputs the
resultant signal to the signal demuliplexer 202. The reception
signal received by the receiver 201 includes the discovery signal
or the control signal and the data signal transmitted from the base
station 100. In a case where information received from the
discovery signal information receiver 203 indicates that a
discovery signal should be detected in a frequency band different
from a reception frequency band of the base station 100, then the
receiver 201 changes the reception frequency band and receives the
signal.
The signal demuliplexer 202 demultiplexes the discovery signal and
the signal for use in the RSSI measurement from the reception
signal received from the receiver 201 based on the discovery signal
information received from the discovery signal information receiver
203, and the signal demuliplexer 202 outputs them to the RSRP/RSRQ
measurement unit 204. Furthermore, the signal demuliplexer 202
extracts signals corresponding to a data resource (that is, a data
signal and a control signal) from the reception signal. The signal
demuliplexer 202 outputs the extracted data signal as a reception
data signal and outputs the extracted control signal to the
discovery signal information receiver 203.
The discovery signal information receiver 203 extracts the
discovery signal information from the control signal received from
the signal demuliplexer 202 and outputs the discovery signal
information to the receiver 201 and the signal demuliplexer 202.
The discovery signal information include information indicating the
transmission repetition period, the transmission subframe the
transmission frequency band, the bandwidth, or RB (Resource Block)
or the like. Note that not only the discovery signal information of
the cell managed by the base station 100 but also discovery signal
information of another cell adjacent to the cell managed by the
base station 100 are included.
The RSRP/RSRQ measurement unit 204 identifies the cell by using the
discovery signal received from the signal demuliplexer 202 and
achieves synchronization with the cell. After the cell
synchronization, the RSRP/RSRQ measurement unit 204 measures RSRP
using a resource in which the discovery signal is mapped, and
measures RSSI using a resource different from the resource in which
the discovery signal is mapped. The RSRP/RSRQ measurement unit 204
then calculates RSRQ using measured RSRP and RSSI. The RSRP/RSRQ
measurement unit 204 outputs the measurement results including RSRP
and RSRQ to the measurement report generator 205.
The measurement report generator 205 generates a measurement report
using RSRP or RSRQ of the base station 100, another base station,
and a transmission point, and outputs the generated measurement
report to the signal assignment unit 206.
The signal assignment unit 206 assigns the transmission data signal
and the measurement report received from the measurement report
generator 205 to a particular resource of an uplink and transmits
them to the transmitter 207.
The transmitter 207 performs a transmission process such as
up-converting on the input signal and transmits it.
Operations of Base station 100 and Mobile station 200
Operations of the base station 100 and the mobile station 200
configured in the above-described manner are described below.
In the present embodiment, an explanation is given below for a case
where PRS (Positioning Reference Signal) already existing in the
LTE-Advanced system is used in the design of the discovery signal.
PRS is a reference signal for use in measuring position information
of a mobile station and is used in measuring timing of downlink
signals from a plurality of base stations.
FIG. 4 illustrates an example of PRS mapping. In a case where the
number of CRS ports is equal to or less than 2 and normal CP
(Cyclic Prefix) is employed, 16 REs (Resource Elements) are mapped
per antenna port in each subframe. Note that 1 RE is a unit of
resource given by 1 subcarrier.times.1 OFDM symbol shown in FIG. 4.
As shown with PRSs #0 to #5 in FIG. 4, 6 shift patterns are defined
according to Cell IDs. Each PRS is scrambled with the cell ID. That
is, in FIG. 4, PRSs #0 to #5 may be transmitted from different
cells.
Furthermore, as illustrated in FIG. 4, CRSs and PRSs are mapped
such that no resource collision occurs.
However, a resource collision is not avoided between DMRS
(Demodulation Reference Signal) and PRS, and thus there is a
possibility that degradation in performance occurs if DMRS is used
when data is transmitted. If it is taken into account a possibility
of an occurrence that discovery signals are transmitted from a
plurality of cells at the same time using the same subframe, it is
desirable to avoid data transmission to increase the accuracy of
detection of the discovery signal. Thus, in the following
description, it is assumed that data (PDSCH) is not transmitted in
a subframe in which PRS is transmitted as a discovery signal.
FIG. 5 illustrates an example of an operation of the base station
100 and that of the mobile station 200 according to the present
embodiment.
For example, the base station 100 generates discovery signal
information by changing transmission parameters set for PRS in
terms of a transmission repetition period, a transmission subframe,
a transmission frequency band, a bandwidth, an RB, or the like. The
base station 100 transmits the discovery signal information as a
control signal of a higher layer to the mobile station 200.
The mobile station 200 is instructed to detect a discovery signal
in a subframe specified by higher-layer signaling (discovery signal
information). The mobile station 200 identifies a cell using PRS
transmitted in the subframe in which the discovery signal is
detected, and achieves synchronization in terms of
time/frequency.
Subsequently, the mobile station 200 measures RSRP and RSSI on the
identified cell.
In this measurement, as illustrated in FIG. 5, the mobile station
200 measures RSRP, that is, reception power of the discovery signal
in the subframe (RSRP measured subframe) in which the discovery
signal is transmitted, and determines the mean value per RE. This
measurement is performed using a resource in which the discovery
signal (PRS) is mapped. For example, in a case where the discovery
signal information indicates information associated with PRS #0,
the mobile station 200 measures RSRP (the average reception power
of 16 REs in which PRS #0 is mapped) using PRS #0.
On the other hand, as illustrated in FIG. 5, the mobile station 200
measures, as RSSI, the average reception power per OFDM symbol in a
subframe (RSSI measured subframe) different from the subframe in
which the discovery signal is transmitted. In FIG. 5, the mobile
station 200 measures the average reception power (RSSI) per OFDM
symbol using all OFDM symbols in the RSSI measured subframe.
As described above, in a subframe in which PRSs used as discovery
signals are mapped densely, it is assumed that no data is allocated
in any cell. Because no data is allocated in any cell in such a
subframe, a mobile station cannot accurately measure traffic from a
cell different from a cell to be measured (that is, amount of
interference to the cell to be measured). Furthermore, in the
subframe in which PRSs from a plurality of cells are densely
mapped, the mobile station measures amount of interference in a
state (OFF state) different from a state (ON state) in which data
is actually transmitted. That is, there is a difference in
interference measured by the mobile station between a subframe in
which a data signal is transmitted and a subframe in which
discovery signals are transmitted.
To handle the above situation, in the present embodiment, the
mobile station 200 measures RSSI using another subframe including
no discovery signal. In this case, a discovery signal transmitted
from a cell in the OFF state is not used in the RSSI measurement
performed by the mobile station 200. In the RSSI measured subframe
shown in FIG. 5, the mobile station 200 is capable of measuring
RSSI such that data from a cell in the ON state is reflected in the
measurement, because data is assigned in another cell (in the ON
state). That is, in a case where discovery signals and data are not
transmitted in the same subframe, the mobile station 200 uses a
subframe including a discovery signal as a subframe for RSRP
measurement and uses a subframe including no discovery signal as a
subframe for RSSI measurement.
In the present embodiment, as described above, the mobile station
200 measures RSRP using a resource in which a discovery signal is
mapped, and measures RSSI using a resource different from the
resource in which the discovery signal is mapped (using a different
subframe in the present embodiment). The mobile station 200 then
calculates RSRQ using RSRP and RSSI.
Thus, a discovery signal transmitted from a cell in the OFF state
is not included in RSSI measurement performed by the mobile station
200. Furthermore, in an RSSI measured area, the mobile station 200
is capable of measuring RSSI such that data from a cell in the ON
state is reflected in the measurement. Thus, the mobile station 200
is capable of measuring RSSI in a resource in which data is
transmitted from a cell in the ON state. That is, the mobile
station 200 is capable of measuring RSSI corresponding to the
traffic without being influenced by the discovery signal. Thus, the
mobile station 200 is capable of accurately measuring RSSI even in
a case where a discovery signal is transmitted. This results in an
increase in RSRQ measurement accuracy, and thus, for example, it
becomes possible to select an optimum cell in the cell selection
according to the RSRQ criterion.
Note that although the present embodiment has been described for a
case where PRS is used as a discovery signal, another existing
signal other than PRS may be used as the discovery signal.
Modifications of First Embodiment
OFDM Symbols Used in Measuring RSSI
If the mobile station 200 measures RSSI for all OFDM symbols in a
subframe in which RSSI is to be measured and determines the average
as shown in FIG. 5, then this results in an increase in the amount
of resource used in the averaging. This makes it possible to
increase the RSSI measurement accuracy.
However, the method of measuring RSSI in RSSI measured subframes is
not limited to that described above, but alternatively, for
example, a restriction may be imposed on OFDM symbols used in the
RSSI measurement.
For example, OFDM symbols used in the RSSI measurement may be
limited to OFDM symbols in which CRSs are mapped. In the case shown
in FIG. 5, OFDM symbols subjected to the RSSI measurement are 4
OFDM symbols, that is, OFDM symbols #0, #4, #7, and #11. In a case
where cells to be subjected to the comparison in the cell selection
are operated using the conventional PSS/SSS/CRS and thus RSSI is
measured using only OFDM symbols in which CRSs are mapped, the
resultant RSSI includes power of CRSs of a plurality of cells.
Therefore, also in cells operating using discovery signals, it is
possible to easily make a cell comparison by measuring RSSI
according to a similar criterion to the criterion employed in cells
operating using PSS/SSS/CRS.
Alternatively, OFDM symbols located in an earlier portion of a
subframe or OFDM symbols located in a latter portion of the
subframe may be excluded from the RSSI measurement. That is, the
OFDM symbols used in the RSSI measurement may be limited to a
particular number of OFDM symbols in the earlier portion of the
subframe or OFDM symbols in the latter portion of the subframe
other than the particular number of OFDM symbols in the earlier
portion of the subframe. An OFDM symbol period excluded from the
RSSI measurement may be used as a gap period. For example, the gap
period may be used as a period in which cell identification,
synchronization, and RRM are performed in switching from a
currently connected band to another band. By providing a gap period
in a subframe used in RSSI measurement, the following advantages
are achieved. The mobile station 200 is allowed to use an adjacent
subframe before or after a subframe used in the RSSI measurement in
order to receive data in the band in connection. Furthermore, in a
case where the mobile station 200 goes into a receiving state only
when DRX is set from a cell in connection and a discovery signal is
detected, then, when the measurement is performed on a cell with
the same band as the band of the cell in connection, it is possible
to reduce the period in which the receiving state is maintained,
and thus it is possible to suppress power consumption.
Subframe Used in RSSI Measurement
As illustrated in FIG. 5, subframes used in the RSSI measurement
may be limited to subframes adjacent to subframes in which
discovery signals are transmitted. The result of this is that
subframes used in RSRP and RSSI are located successively, and thus
it becomes possible to reduce the time taken to measure RSRP and
RSSI. The reduction in the measurement time makes it possible to
reduce a time period in which data transmission from a cell in
connection is stopped during the detection of a new cell, which
makes it possible to reduce power consumption of the mobile station
200.
Alternatively, subframes used in the RSSI measurement may be
limited to particular types. For example, subframes used in the
RSSI measurement may be limited to Multimedia broadcast multicast
service Single Frequency Network (MBSFN) subframes. No existence of
CRS allows each MBSFN subframe to include a corresponding extra
amount of data (PDSCH), which makes it possible for a traffic in a
band of a cell subjected to RSSI measurement to be easily reflected
in a result of RSSI measurement. This is because small cells have
no significant interference from CRS and there is a high
probability that MBSFN subframes, in which it is easy to use DMRS,
are used in data transmission.
Conversely, the subframes used in the RSSI measurement may be
limited to non MBSFN subframes. In this case, many OFDM symbols
including CRSs appear in the subframes used in the RSSI
measurement, and thus the RSSI measurement value may include
interference of CRS of a cell in the ON state. In particular, in a
case where OFDM symbols used in the RSSI measurement are limited to
OFDM symbols in which CRSs are mapped, RSSI is measured in non
MBSFN subframes. This results in an increase in the number of OFDM
symbols used in the averaging. Furthermore, in a case where a cell
under comparison in the cell selection is operating using
conventional PSS/SSS/CRS and thus in a case where the RSSI
measurement is performed with the limitation to OFDM symbols in
which CRSs are mapped, the following advantage is obtained. The
RSSI measurement condition for the cell of interest is similar to
that for cells operating using discovery signals, which makes it
easier to perform the comparison in the cell selection.
Second Embodiment
The first embodiment has been described for a case where PRS is
used as a discovery signal. On the other hand, in a second
embodiment described below, CSI-RS is used as a discovery signal or
PSS/SSS/CRS (reduced PSS/SSS/CRS) with a low transmission rate is
used.
In the present embodiment, a base station and a terminal are
basically similar in configuration to the base station 100 and the
terminal 200 according to the first embodiment, and thus FIG. 2 and
FIG. 3 are also used in the following description.
Now detailed descriptions are given below for a case (first example
of operation) in which CSI-RS is used as a discovery signal, and a
case (second example of operation) in which PSS/SSS/CRS is
used.
First Example of Operation
CSI-RS is a reference signal used in CSI measurement. For example,
as illustrated in FIG. 6, in a case where 2 REs per antenna port
are allocated with a CDM multiplex level of 2 in Normal CP, it is
designed such that it is allowed to orthogonally allocate CSI-RSs
for 40 antenna ports per subframe. Furthermore, CSI-RS is scrambled
with a cell ID. Locations of CRSs and DMRS are designed such that
no collision occurs in terms of locations, and thus PDSCH, which is
a downlink data signal, is allowed to be mapped in the same
subframe in which CSI-RS is mapped. By notifying the mobile station
200 in advance of REs of CSI-RS used for discovery signals, it
becomes possible to allocate PDSCH so as not to be located in REs
in which CSI-RS is mapped.
Note that in the mobile station 200, REs to which discovery signals
are supposed to be mapped in the reception of PDSCH may be all REs
(in Normal CP) to which it is allowed to map discovery signals.
Alternatively, mapping of discovery signals may be limited to
locations of candidates, specified by the higher-layer signaling,
for discovery signals.
As in the first embodiment, the base station 100 generates
discovery signal information by changing transmission parameters
set for CS-RS in terms of a transmission repetition period, the
transmission subframe the transmission frequency band, a bandwidth,
an RB, or the like. The base station 100 transmits the discovery
signal information, as a higher layer control signal, to the mobile
station 200.
As in the first embodiment, the mobile station 200 is instructed to
detect a discovery signal in a subframe specified by higher-layer
signaling (discovery signal information). The mobile station 200
identifies a cell using CSI-RS transmitted in a subframe in which a
discovery signal is detected, and achieves synchronization in terms
of time/frequency.
Subsequently, the mobile station 200 measures RSRP and RSSI on the
identified cell.
In this measurement, the mobile station 200 measures, as RSRP,
reception power of the discovery signal using REs in which
discovery signals (CSI-RS) are mapped, and determines the mean
value per RE.
Furthermore, the mobile station 200 measures, as RSSI, the average
reception power per OFDM symbol in the same subframe in which the
discovery signal is transmitted.
More specifically, the mobile station 200 measures RSSI in a
subframe subjected to the RSSI measurement by using REs different
from REs in which discovery signals (CSI-RS) are mapped.
For example, as illustrated in FIG. 7, the mobile station 200 may
measure RSSI using only part of OFDM symbols other than OFDM
symbols each including an RE in which a discovery signal (CSI-RS)
is mapped wherein the part of OFDM symbols each include CRS. As in
the first embodiment, cells to be subjected to the comparison in
the cell selection are operated using the conventional PSS/SSS/CRS,
and thus in a case where RSSI is measured using only OFDM symbols
in which CRSs are mapped, the resultant RSSI includes power of CRSs
of a plurality of cells. Therefore, also for a cell operating using
a discovery signal, it become possible to easily perform comparison
in cell selection by measuring RSSI according to a similar
criterion to that employed in a cell operating using
PSS/SSS/CRS.
Alternatively, as illustrated in FIG. 8, without imposing
restrictions on OFDM symbols used in the RSSI measurement, the
mobile station 200 may measure RSSI using all REs other than REs in
which discovery signals are mapped. The increase in the number of
OFDM symbols used in the RSSI measurement results in an increase in
the amount of resource used in the averaging, and thus it becomes
possible to increase the RSSI measurement accuracy.
Note that in the example shown in FIG. 8, some of OFDM symbols used
in the RSSI measurement include CSI-RS. However, OFDM symbols
including CSI-RS may be excluded from the RSSI measurement. That
is, the mobile station 200 may measure RSSI using such OFDM symbols
other than OFDM symbols each including an RE in which a discovery
signal (CSI-RS) is mapped. For example, in Normal CP in FIG. 8, the
mobile station 200 does not use OFDM symbols #5, #6, #9, #10, #12,
and #13 in the RSSI measurement but the mobile station 200 measures
RSSI using only the other OFDM symbols. When part of REs in OFDM
symbols are used in the RSSI measurement, it is necessary to
determine the average per OFDM symbol taking into account the
number of REs used in the measurement. In contrast, in the case
where OFDM symbols including CSI-RS are excluded from the RSSI
measurement, RSSI is averaged and given as a power measurement
value per OFDM symbol, and thus it becomes possible to easily
calculate the average.
In the first example of operation, as described above, subframes
used in the RSSI measurement may be the same as the subframes in
which discovery signals (CSI-RS) are transmitted. This makes it
possible for the mobile station 200 to measure RSRP and RSSI in the
same subframe, and thus it is possible to reduce the measurement
time.
Second Example of Operation
In the case of Normal CP, PSS is mapped to the OFDM symbol #6 in
FDD, while PSS is mapped to the OFDM symbol #2 in TDD. SSS is
mapped to the OFDM symbol #5 in FDD, while SSS is mapped to the
OFDM symbol #13 in a subframe immediately before a subframe to
which PSS is mapped. PSS/SSS is designed so as to avoid a collision
with CRS, and thus it is allowed to allocate PDSCH, which is a
download data signal, in the same subframe. It is possible to
allocate PDSCH so as not to be located in any OFDM symbol in which
PSS/SSS is mapped. In the case of Normal CP, when the number of
antenna ports for CRS is equal to or less than 2, CRS is mapped to
OFDM symbols #0, #4, #7, and #11, while when the number of antenna
ports for CRS is equal to 4, CRS is also mapped to OFDM symbols #1
and #8.
If the number of CRS antenna ports for the discovery signal is set
to 2 per cell, then, as illustrated in FIG. 9, there are three
CRS-to-RE mapping patterns different depending on cell IDs. In a
case where the number of CRS antenna ports for the discovery signal
is set to 1 per cell, then there are six CRS-to-RE mapping patterns
different depending on cell IDs.
As in the first embodiment, for example, the base station 100
generates discovery signal information by changing transmission
parameters set for PSS/SSS/CRS in terms of a transmission
repetition period, a transmission subframe, a transmission
frequency band, a bandwidth, an RB, or the like. The base station
100 transmits the discovery signal information as a higher-layer
control signal to the mobile station 200.
As in the first embodiment, the mobile station 200 is instructed to
detect a discovery signal in a subframe specified by higher-layer
signaling (discovery signal information). The mobile station 200
identifies a cell using PSS/SSS/CRS transmitted in a subframe in
which a discovery signal is detected, and achieves synchronization
in terms of time/frequency.
Subsequently, the mobile station 200 measures RSRP and RSSI on the
identified cell.
In this measurement, the mobile station 200 measures, as RSRP,
reception power of the discovery signal using REs in which
discovery signals (CRS) are mapped, and determines the mean value
per RE.
Furthermore, the mobile station 200 measures, as RSSI, the average
reception power per OFDM symbol in the same subframe in which the
discovery signal is transmitted.
More specifically, the mobile station 200 measures RSSI in a
subframe subjected to the RSSI measurement by using REs different
from REs in which discovery signals (PSS/SSS/CRS) are mapped.
Note that in the second example of operation, CRS is used as the
discovery signal, and thus CRS is transmitted not only in cells in
the ON state but also in cells in the OFF state. Therefore, the
second example of operation is different from the first example of
operation in that in subframes in which discovery signals are
transmitted, CRS is excluded from the RSSI measurement.
For example, for subframes in which discovery signals (PSS/SSS/CRS)
are transmitted, the mobile station 200 may measure RSSI in a
manner described below with reference to FIG. 10. That is, the
mobile station 200 may measure RSSI using OFDM symbols other than
OFDM symbols in which discovery signals are mapped (that is, using
OFDM symbols in which no discovery signal is mapped). For example,
in a case where RSSI is measured using the same subframe as the
discovery signal, RSSI is measured as follows depending on a
situation described below. In Normal CP, when FDD is employed and
the number of CRS antenna ports for the discovery signal is set to
2 per cell then, as illustrated in FIG. 10, the mobile station 200
excludes OFDM symbols #0, #4, #5, #6, #7, and #11 from the RSSI
measurement and measures RSSI using OFDM symbols #1, #2, #3, #8,
#9, #10, #12, and #13. When RSSI is measured using the same
subframe as the subframe in which the discovery signal is
transmitted, it is possible to measure RSRP and RSSI in the same
subframe and thus it is possible to reduce the measurement
time.
FIG. 11 illustrates another example of a method of measuring RSSI
in the second example of operation. As illustrated in FIG. 11, the
mobile station 200 measures RSSI using a subframe in which no
discovery signal is transmitted. Furthermore, as illustrated in
FIG. 11, in subframes used in RSSI measurement, the mobile station
200 performs the RSSI measurement using only OFDM symbols in which
CRS is mapped. Thus, as in the first embodiment, and as in the
first example of operation, also for a cell operating using a
discovery signal, it become possible to easily perform comparison
in cell selection by measuring RSSI according to a similar
criterion to that employed in a cell operating using
PSS/SSS/CRS.
The RSSI measurement has been described above fore the case (the
first example of operation) in which CSI-RS is used as the
discovery signal and for the case (the second example of operation)
in which PSS/SSS/CRS is used as the discovery signal.
In the present embodiment, as described above, the mobile station
200 measures RSRP using a resource in which a discovery signal is
mapped, and measures RSSI using a resource different from the
resource in which the discovery signal is mapped. The mobile
station 200 then calculates RSRQ using RSRP and RSSI. The resource
is, for example, an RE, an OFDM symbol, or a subframe.
Thus a discovery signal transmitted by a cell in the OFF state is
not subjected to the RSSI measurement performed by the mobile
station 200. In the RSSI measured area, the mobile station 200 is
capable of measuring RSSI such that data from a cell in the ON
state is reflected in the measurement, that is, the mobile station
200 is capable of measuring RSSI in a resource used by a cell in
the ON state to transmit data. That is, the mobile station 200 is
capable of performing RSSI measurement depending on traffic without
being influenced by discovery signals. Thus, as in the first
embodiment, the mobile station 200 is capable of accurately
measuring RSSI even in a case where a discovery signal is
transmitted. This results in an increase in RSRQ measurement
accuracy, and thus, for example, it becomes possible to select an
optimum cell in cell selection according to the RSRQ criterion.
The present disclosure has been described with reference to
embodiments.
Other Embodiments
[1] In the embodiments described above, a mobile station does not
necessarily detect all subframes in which discovery signals are
transmitted. Therefore, in the embodiments described above,
"subframes in which discovery signals are transmitted" may be
replaced by "subframes specified to be used in measuring discovery
signals".
[2] The design of discovery signals is not limited to PRS, CSI-RS,
and PSS/SSS/CRS described above. The design may be made differently
such that discovery signals are not used in the RSSI measurement as
in the embodiments described above.
[3] In the embodiments described above, explanations have been
given by way of example for a case where RSSI is measured in
subframes different from those in which discovery signals are
mapped. However, depending on a case, there is a possibility that a
frequency band in which a discovery signal is transmitted is
limited to a part of a frequency band of a cell. In this case,
instead of measuring RSSI in subframes different from those in
which discovery signals are mapped, RSSI may be measured in a
frequency band and RB in which no discovery signal is mapped.
[4] In the embodiments described above, it is assumed by way of
example that the present disclosure is implemented using hardware.
The present disclosure may be implemented using software in
cooperation with hardware.
The functional blocks used in the explanation of the embodiments
described above may be typically realized by an LSI, which is an
integrated circuit. They each may be realized on one chip
individually, or they may all be integrated on one chip. Note that
the LSI may be an integrated circuit called an IC, a system LSI, a
super LSI, or an ultra LSI depending on the integration scale.
Furthermore, the implementation using the integrated circuit is not
limited to that using the LSI, but the implementation may be
realized using a dedicated circuit or a general-purpose processor.
Alternatively, FPGA (Field Programmable Gate Array) which is
allowed to be programmed after the LSI is produced or
reconfigurable processor which is reconfigurable in terms of
circuit cell connections in the LSI or setting thereof may be
used.
When a further advance is made in semiconductor technology or
derivative technology and, as a result, integrated circuit
technology appears that will replace the LSI, as a matter of
course, functional blocks may be integrated using such technology.
A possibility of such technology is biotechnology or the like.
In the present disclosure, a mobile station may include a receiver
that receives a discovery signal, and a measurement unit that
measures Reference Signal Reception Power (RSRP) using a resource
in which the discovery signal is mapped, measuring Received Signal
Strength Indicator (RSSI) using a resource different from the
resource in which a discovery signal is mapped, and calculates
Reference Signal Reception Quality (RSRQ) using RSRP and RSSI.
In the mobile station according to the present disclosure, the
resource may be a subframe and the measurement unit may measure the
RSSI in a second subframe different from a first subframe in which
the RSRP is measured.
In the mobile station according to the present disclosure, the
measurement unit may measure the RSSI using a plurality of all
symbols included in the second subframe.
In the mobile station according to the present disclosure, the
measurement unit may measure the RSSI using those symbols in the
plurality symbols included in the second subframe that include Cell
specific Reference Signal (CRS).
In the mobile station according to the present disclosure, the
second subframe may be located adjacent to the first subframe.
In the mobile station according to the present disclosure, the
measurement unit may measure the RSSI using symbols that are part
of the plurality of symbol included in the subframe in which the
RSSI is measured but that are other than a particular number of
symbols in an earlier part or a latter part in the subframe in
which the RSSI is measured.
In the mobile station according to the present disclosure, the
resource may be a subframe and the measurement unit may measure the
RSSI in the same subframe as the subframe in which the RSRP is
measured.
In the mobile station according to the present disclosure, the
resource may be a resource element (RE) and the measurement unit
may measure the RSSI using an RE other than any RE in which a
discovery signal is mapped.
In the mobile station according to the present disclosure, the
measurement unit may measure the RSSI using a symbol other than any
symbol including a resource in which a discovery signal is
mapped.
In the mobile station according to the present disclosure, the
measurement unit may measure the RSSI using a symbol that is other
than any symbol including a resource in which a discovery signal is
mapped and that includes Cell specific Reference Signal (CRS).
The present disclosure provides a reception quality measurement
method including the steps of receiving a discovery signal at the
mobile station, and measuring Reference Signal Reception Power
(RSRP) using a resource in which the discovery signal is mapped,
measuring Received Signal Strength Indicator (RSSI) using a
resource different from the resource in which a discovery signal is
mapped, and calculating Reference Signal Reception Quality (RSRQ)
using RSRP and RSSI.
The present disclosure is useful for a mobile communication
system.
* * * * *